/ 10 July 2023

Invasive fish, snails found in Kruger National Park

Carps In The Landing Net: Fishing Farm
Australian crayfish, carp from Europe and Asia, silver carp from China, Nile tilapia, the North American largemouth bass and mosquitofish, suckermouth catfish from South America and alien snails have invaded the park

In February 2016, the first record was received of the invasive Australian redclaw crayfish in the Crocodile River from the Kruger National Park.

Since then, it’s been reported from a number of different localities, said Llewellyn Foxcroft, a SANParks scientist in invasion ecology at Kruger and a member of the Centre for Invasion Biology at Stellenbosch University. 

“There’s now some confirmed records in the Sand River outside the park,” said Foxcroft, who was speaking at a recent webinar hosted by the SANParks Honorary Rangers on managing invasive species in Kruger. 

“Then also in the upper Letaba catchment and, potentially, not too far from the Sabie River outside the park,” adding that recent surveys found the redclaw in other areas, suggesting the species is “establishing”.

The crayfish, with its fan-like tail and large pincers, may reach a length of 40cm and weigh 2.5kg, according to Invasives South Africa. Its brown to dark grey and the pincers are steel blue with bright red patches, particularly on the outer sides. 

“These crayfish are not indigenous to South Africa and may negatively impact indigenous freshwater species. It also carries parasites, which have further impacts on indigenous species,” it said.

Swimming into trouble

Foxcroft said the invasive common carp has also been reported in Kruger or at least in some of the boundary rivers. 

The common carp, which originates in Europe and Asia, has significant negative effects on the environment, including competition with other organisms for the same food sources causing biodiversity loss, according to Invasives South Africa.

“I’m not sure if it’s established, these are reports that we’ve got, but the silver carp, we know, has established very well in the Olifants River,” said Foxcroft. 

Silver carp, which originates from China, is another invasive species that competes with indigenous species for habitat and resources. 

Foxcroft said silver carp had been in Massinger Dam in Mozambique, which is fed by the Olifants River, for many years and “pushes back into the park right up through near the weir or just below the Olifants rest camp. So, those fish are definitely established and invasive in that area.”

Foxcroft said the Nile tilapia had been reported from other rivers for a number of years, and the concern is that they, too, will become established. It originates from Central and North Africa and the Middle East.

“They also have different behaviour to some of the local tilapia but one of the concerns is also the hybridisation with the Mozambique tilapia, resulting in the loss of the genetically pure tilapia species. And there have been reports of possible hybrid tilapia caught from nearby the Phabeni area of the park. So, there are early signs now that this might be taking place.”

Invasive bass, mosquitofish

Bass were first recorded in 2019 near Phabeni in the southern region of Kruger. “Whether they’re established in that area or perhaps established upstream … and making their way down to the park, how far they will go into the park is not certain at this stage,” said Foxcroft. 

“It wasn’t too much later that largemouth bass was found in the Crocodile River, coming in from potentially upstream or in any one of the dams or streams that feed into the park from the southern region.” 

Largemouth bass is a predator from North America, with no competition, said Invasives South Africa. It pushes out indigenous species and could cause their extinction.

Foxcroft said there were reports in 2020 of mosquitofish at Balule weir on the Olifants River. “We’re not sure, but there were a number of these fish caught there, so personally I wouldn’t have expected them to establish in an area where I would have thought there are so many predators that would have an impact on them. To have got to that point in the park means they would have been able to avoid some level of predation and breed up to some level.” 

Mosquitofish, indigenous to the Mississippi River, are listed as one of the world’s worst invasive species. “Adults are extremely aggressive and attack other fish, shredding their fins and sometimes killing them,” according to Invasives South Africa.

The suckermouth catfish have been recorded in the Crocodile River. These are commonly kept in aquariums,“but the problem is when they get too big and then people need to release them and they go to the nearest stream or river to set them free”. “Now, whether it was an individual fish that has escaped or been released [ in the Crocodile River] and it was still surviving or if there are others, a bigger population, we’re not sure, but the fact it was found there is a sign of some concern.”

Pesky alien snails

Foxcroft said there are also a number of alien snail species that have invaded Kruger, the worst of which is the Tarebia snail, which occurs widely across the park and invaded it probably from the early 2000s. 

“And one of the problems is just the sheer volumes of snails that build up in an area. And with eutrophication and a lot of algae building it up, it also suits them because it gives them habitat or food,” he said. “Many years ago, I was walking along the Crocodile River, where it was like walking along the coast, along the seashore, walking on shells.”

On monitoring and detecting alien animal species in Kruger, Foxcroft said many of the new invasive species are fish “so there’s not a huge amount we can do in terms of managing” these species.

“But what we do need to understand is where do they occur in the park, where they are spreading, where are the threats and, over time, try to understand the impacts they might be having on the ecosystem and other biodiversity in the park.”